Speed improvement for opening the movies library

On all but the first time of opening the movies library, the CFileItemList
is built using CArchive. A large proportion of the time taken to do this is
actually spent in CVideoInfoTag::Archive() where it calls
CScraperUrl::Parse() and CFanart::Unpack(). Unless I've overlooked something,
these calls are unnecessary - the XML parsing they do is not performed the
first time the movies library is opened (when instead the virtual directory
videodb://movies/titles/ is enumerated using a database query). Also, the
Parse() method is called again when you do something like open the Movie
Information window, irrespective of whether it was called when the library
was opened.

Also note, it is unusual for any implementation of IArchivable::Archive() to
do anything other than read or write a data member from/to a CArchive, so
these calls to Parse() and Unpack() feel like an anomaly.

Additionally, inserted a call to std::vector::reserve() before reading the
cast list, because it was an obvious speed win.

Here are some benchmarks for the function CGUIMediaWindow::GetDirectory()
on a library of 600 movies, running on a Raspberry Pi:
* First call (before or after):    3.4 seconds
* Subsequent calls (before patch): 7.3 seconds
* Subsequent calls (after patch):  2.9 seconds
Merge pull request #3234 from fritsch/aefixes2

ActiveAESink: More debugging
[atv2] - fix  bt keyboard support (fixes space, tab, esc, enter and modifier keys)
Merge pull request #3235 from Memphiz/atv2bt

[atv2] - fix  bt keyboard support (fixes space, tab, esc, enter and modi...
English spelling corrections English/strings.po
Merge pull request #3239 from readmanr/english-check

English spelling corrections English/strings.po
linuxrenderer: do not render if upload texture fails
vdpau: redesign
vdpau: make interop gl default and remove setting, rename and intvert interop yuv
vdpau: observe ffmpeg tags for color space
vdpau: switch off de-interlacing on ff
linuxrenderer: drop method RenderMultiPass
linuxrenderer: implement progressive weave for vdpau
vdpau: set deinterlacing method to auto, if default method not supported
vdpau: calculate timestamp of second field when doing deinterlacing
vdpau: comment some features that will be added later
renderer: protect against div by zero
vdpau: add fence for detecting when a buffer can be reused
Merge pull request #3037 from FernetMenta/vdpau

VDPAU redesign
ffmpeg: add swr_set_compensation to interface